mutex_instances table lists
all the mutexes seen by the Performance Schema while the
server executes. A mutex is a synchronization mechanism used
in the code to enforce that only one thread at a given time
can have access to some common resource. The resource is said
to be “protected” by the mutex.
When two threads executing in the server (for example, two user sessions executing a query simultaneously) do need to access the same resource (a file, a buffer, or some piece of data), these two threads compete against each other, so that the first query to obtain a lock on the mutex causes the other query to wait until the first is done and unlocks the mutex.
The work performed while holding a mutex is said to be in a “critical section,” and multiple queries do execute this critical section in a serialized way (one at a time), which is a potential bottleneck.
mutex_instances table has
The instrument name associated with the mutex.
The address in memory of the instrumented mutex.
When a thread currently has a mutex locked,
THREAD_IDof the locking thread, otherwise it is
TRUNCATE TABLE is not permitted
For every mutex instrumented in the code, the Performance Schema provides the following information.
setup_instrumentstable lists the name of the instrumentation point, with the prefix
When some code creates a mutex, a row is added to the
OBJECT_INSTANCE_BEGINcolumn is a property that uniquely identifies the mutex.
When a thread attempts to lock a mutex, the
events_waits_currenttable shows a row for that thread, indicating that it is waiting on a mutex (in the
EVENT_NAMEcolumn), and indicating which mutex is waited on (in the
When a thread succeeds in locking a mutex:
events_waits_currentshows that the wait on the mutex is completed (in the
The completed wait event is added to the
mutex_instancesshows that the mutex is now owned by the thread (in the
When a thread unlocks a mutex,
mutex_instancesshows that the mutex now has no owner (the
When a mutex object is destroyed, the corresponding row is removed from
By performing queries on both of the following tables, a monitoring application or a DBA can detect bottlenecks or deadlocks between threads that involve mutexes:
events_waits_current, to see what mutex a thread is waiting for
mutex_instances, to see which other thread currently owns a mutex